Implantable medical devices, such as stents, stent grafts, vena cava filters, occlusion devices, embolisation coils and so on, are often deployed endoluminally through a suitable percutaneous entry point of a patient. For instance, a medical device to be fitted within the aorta may be passed via the femoral artery, thus from an entry point a substantial distance from the treatment site.
The introducers used for the deployment of such devices tend to be elongate catheter assemblies in which the medical device is held at a distal end of the introducer in a radially compressed state until it is ready to be deployed. Typically, such introducer assemblies are formed from a plurality of components including one or more catheters, device restraining elements, guide wires and so on. The introducer also includes a covering sheath which covers the components of the introducer as well as the medical device.
Once the introducer has been inserted into a patient and the distal end thereof carrying the medical device located at the site of a patient's vasculature at which the device is to be positioned, the deployment mechanism is operated. The first stage of this is to pull back the outer sheath so as to expose the medical device, whereupon the constraining mechanism holding the medical device to the carrier catheter is then activated, normally in stages, to release the device. In some cases the device is releasable by self-expansion while in other cases a separate expansion mechanism, such as a balloon, is used.
In order to maintain the integrity of the components carried in the introducer, as well as to keep the outer diameter of the introducer, particularly at its distal end, as small as possible, the components of the assembly tend to be a relatively tight fit with one another. This relatively tight fit results in friction between the various slidable components of the assembly. To add to this, the proximal end of the device, that is the end which remains outside the patient during the procedure and which includes the various manual controls used by the clinician, is provided with at least one haemostatic valve or seal, often several, to prevent undesired loss of bodily fluids during the operation. There is necessarily provided a seal or valve between the covering sheath and the inner catheter or catheters to prevent fluid leakage from the space between these components. Such seals tend to contribute to friction between the various sliding components of the device and thus to an increased required operation force.
Moreover, introducer devices of this nature tend to be very long, an introducer for a medical device to be implanted in the aorta or heart typically having a length of at least 1.5 metres. Furthermore, the path such introducers have to follow through a patient's vasculature is normally tortuous, such that they have to curve and often twist along their length. Such curvature and twisting also adds to the friction between the slidable components of the assembly.
The above-mentioned factors lead to it being necessary for the clinician to apply a substantial force to the assembly in order, in particular, to pull the outer sheath back so as to expose the medical device. Having to use a large force to operate the assembly can cause practical difficulties, not only in the effort which the clinician must expend in operating the device, but also in augmenting the risk of the introducer assembly, particularly its distal end, moving unintentionally out of position. Any such movement can lead to incorrect positioning of the medical device. Furthermore, in the case of a device which is rotationally dependant, such as a fenestrated or branched stent graft, for example, any shifting of the introducer can cause rotational misalignment. In order to seek to mitigate these problems, the clinician may need to carry out positional adjustments to the assembly at the same time as effecting the various steps required to deploy the device, which is a complex and time consuming task. At worst, the device cannot be properly deployed and the procedure must be aborted.
There are devices which effect the deployment of the medical device, in particular retraction of the outer sheath, automatically, for instance by means of a sprung loaded and trigger actuated handle assembly. Some rotary handles are known which use a screw mechanism to overcome the longitudinal pulling force; as well as ratchet type handles. These devices can overcome the difficulties indicated above, by providing what could be described as motorised assistance, allowing the clinician to maintain accurate control of the positioning of the introducer while the device exerts the effort required to retract the sheath.
However, in many instances clinicians do not wish to lose the manual control of the operation of the introducer assembly as this can lead to a loss of the fine adjustments clinicians often desire during deployment of the medical device. For instance, a clinician may wish to slow or even temporarily stop the withdrawal of the sheath part way in order to carry out a fine adjustment of the position or orientation of the medical prosthesis. An automatic deployment device may not provide for this. Furthermore, a manually operated introducer gives the clinician tactile feedback as to how the deployment is progressing, whereas an automatic deployment mechanism will generally fail to provide such feedback, causing the clinician to have to rely upon imaging only.
As a result, it in some circumstances it is preferred to retain manual operation of the introducer, despite the problems associated with this.